Process for producing an endless belt

ABSTRACT

A method for producing an endless belt for electrophotography by melt-extruding a thermoplastic resin having a diphenyl sulfone structure represented by the following Formula (1) from a circular die to produce the endless belt continuously

[0001] This application is a division of application Ser. No.09/467,986, filed on Dec. 21, 1999, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to an endless belt for electrophotography,such as an intermediate transfer belt, a transfer material carrying beltor a photosensitive belt, a process for its production, and an imageforming apparatus making use of it.

[0004] 2. Related Background Art

[0005] Intermediate transfer belts, transfer material carrying belts andphotosensitive belts are known as endless belts for electrophotography.

[0006] Compared with image forming apparatuses in which images aretransferred from a first image bearing member onto a second imagebearing member (transfer material) fastened or attracted onto a transferdrum (e.g., Japanese Patent Application Laid-Open No. 63-301960), imageforming apparatuses making use of intermediate transfer belts have anadvantage in that a variety of second image bearing members can beselected without regard for their width and length, including thin paper(40 g/m² paper) and up to thick paper (200 g/m² paper) such asenvelopes, post cards and labels. This is because any processing orcontrol (e.g., the transfer material is held with a gripper, attracted,and made to have a curvature) is not required for the second imagebearing member transfer material.

[0007] Image forming apparatus are also proposed which have a pluralityof recording assemblies in which electrostatic latent images are formedon electrophotographic photosensitive members, the electrostatic latentimages formed are developed and the developed images are transferred toa transfer material, where a full-color image is formed by transferringindividual color toner images superimposingly to the transfer materialwhile transporting it sequentially to the respective recordingassemblies by means of a transfer material carrying belt.

[0008] It is also known to set up electrophotographic photosensitivemembers themselves in the form of endless belts for the purpose ofachieving higher process speed or, especially in image forming apparatushaving a plurality of developing assemblies and others, for the purposeof attaining the freedom in designing the arrangement of developingassemblies and others.

[0009] Image forming apparatus such as copying machines and printersmaking use of endless belts have various advantages as stated above. Onthe other hand, they also have some subjects for improvement.

[0010] For example, intermediate transfer belts are required to have asurface area not smaller than the image region, so that they arenecessarily large in size and also required to have various propertiessuch as resistance properties and surface properties, tending to resultin a high production cost. They also have not necessarily a sufficientdurability and tend to have to be frequently changed to new ones. As theresult, this may raise the main-body price and running cost of copyingmachines and printers and also it may take more time and labor for theirmaintenance. In particular, because of market trends in recent years, ithas increasingly become important to achieve lower prices and providemaintenance-free articles.

[0011] In addition, in order to form good color images, some problemsmust be solved which may occur because a plurality of colors aresuperimposed on the intermediate transfer belt.

[0012] One of them is a misregistration occurring between individualcolor images. In fine lines and characters, even a slight colormisregistration tends to be conspicuous to provide a possibility ofdamaging image quality. The intermediate transfer belt is set across aplurality of shafts and is driven and rotated around them, where thetension applied to every part of the intermediate transfer belt is notnecessarily uniform. Hence, the intermediate transfer belt may undergo alocal elongation and, concurrent therewith, may cause a delicatelyuneven rotation. These are considered to come out as delicate colormisregistration.

[0013] Another problem is occurrence of spots around line images.

[0014] A color image is formed by superimposing a plurality of tonerimages and hence has a larger quantity of toners per unit area than amonochromatic image. Especially in characters or letters and fine lines,toners are present in a large quantity on narrow lines. Moreover,individual color toners have electric charges with the same polarity andhence repulse one another electrostatically. Thus, they can be said tolie on the intermediate transfer belt in an unstable state.

[0015] Meanwhile, because of a difference in arcs drawn by the outersurface and inner surface of the intermediate transfer belt, producedwhen it passes the shafts over which it is set, the intermediatetransfer belt elongates in the peripheral direction at its surface andin the vicinity thereof.

[0016] Thus, the toner images standing unstable and weak to externaldisturbance as stated above are disordered when the intermediatetransfer belt passes the shafts, so that the spots around line imagescome to occur, as so considered.

[0017] Still another problem is half-tone image transfer performance.Faulty images tend to occur when the intermediate transfer belt hasuneven resistance or uneven thickness.

[0018] In addition to these, the intermediate transfer belt alwaysundergoes a tension and a repeated flexural elongation stress, and hencethe intermediate transfer belt is required to have a material rigidityhigh enough to neither break nor crack even when used over a long periodof time. The intermediate transfer belt made of resin also tends tocause what is called a creep, in which the above stress makes the beltelongate gradually with time in the peripheral direction. Any greatchange in size caused by the creep may make a difference from theoriginal designing to aggravate color misregistration or may causefaulty images such as uneven halftone images. It may also cause adifficulty in the rotation of the intermediate transfer belt, acting asa great factor to shorten the life of the intermediate transfer belt.

[0019] For the achievement of cost reduction, which is another importantsubject, the intermediate transfer belt must be made thin-gage in orderto reduce the quantity of materials constituting the belt, and also aproduction process having a smaller number of steps must be provided.Making the belt thin-gage also has the effect of less causing a transfertoner scatter and is an effective means, but on the other hand tends tocause a problem also in respect of durability.

[0020] Moreover, it is essential for the intermediate transfer belt tobe provided, in its neighborhood, with a mechanism of applying a highvoltage. Accordingly, as constituent materials therefor, high-safetymaterials are preferred that may fire or smoke with difficulty againstany unforeseen accidents such as abnormal discharge and insulationfailure.

[0021] However, satisfying all of these high image quality, highdurability, low cost and safety involves technical difficulties.Accordingly, studies are made on intermediate transfer belts made ofresin which satisfy these characteristics at a higher level.

[0022] As for the transfer material carrying belt, it is not the casethat images are directly transferred onto the belt. However, in order toachieve a high image quality, the transfer material carrying belt isrequired to satisfy the same characteristics as those for theintermediate transfer belt, e.g., uniform resistance, surfaceproperties, cost reduction, durability and safety. The same also appliesto the photosensitive belt, on the surface of which images are directlyformed.

[0023] Various processes for producing endless belts used in theintermediate transfer belts and so forth are already known in the art.For example, Japanese Patent Application Laid-Open No. 3-89357 and No.5-345368 disclose a process for producing a semiconducting belt byextrusion. Japanese Patent Application Laid-Open No. 5-269849 alsodiscloses a process in which a belt is obtained by joining both ends ofa sheet to bring it into a cylindrical form. Japanese Patent ApplicationLaid-Open No. 9-269674 discloses a process in which a belt is obtainedby forming a multi-layer coating film on a cylindrical substrate andfinally removing the substrate. Also, Japanese Patent ApplicationLaid-Open No. 5-77252 discloses a seamless belt obtained by centrifugalmolding.

[0024] However, e.g., in the extrusion, the production of a thin-layerbelt which enables reduction of cost and prevention of spots around lineimages involves considerable difficulties when the die gap of anextrusion die is merely set in the same size as the desired beltthickness to carry out extrusion. Even if possible, such extrusion tendsto cause uneven thickness and, as an effect thereof, uneven electricalresistance. In the case when both ends of a sheet are joined, thedifference in height and decrease in tensile strength at the joint tendto come into question. Also, processes making use of solvents as in castmolding, the coating and centrifugal molding require many steps ofpreparing a coating solution, coating the solution and removing thesolvent, resulting in a high cost.

SUMMARY OF THE INVENTION

[0025] The present inventors propose a novel endless belt forelectrophotography which has solved the problems discussed above and isdifferent from conventional ones.

[0026] An object of the present invention is to provide an endless beltfor electrophotography which is producible at a low cost and through asmall number of steps and is rich in variety, and a process for itsproduction.

[0027] Another object of the present invention is to provide an endlessbelt for electrophotography, and an image forming apparatus, which canobtain good color images with less color misregistration and less spotsaround line images.

[0028] Still another object of the present invention is to provide anendless belt for electrophotography which can be free from any changesin size and characteristics of the belt even with its repeated use and,after such use, can maintain the same characteristics as those at theinitial stage, and to provide a process for its production and an imageforming apparatus having such an endless belt.

[0029] The present invention provides an endless belt forelectrophotography which is obtainable continuously by melt extrusionfrom a circular die; the endless belt comprising a layer containing athermoplastic resin having a diphenyl sulfone structure represented bythe following Formula (1)

[0030] The present invention also provides a process for producing anendless belt for electrophotography; the process comprising the step ofmelt-extruding a thermoplastic resin having a diphenyl sulfone structurerepresented by the following Formula (1), from a circular die to producethe endless belt continuously

[0031] The present invention still also provides an image formingapparatus for electrophotography comprising

[0032] an endless belt which is obtainable continuously by meltextrusion from a circular die;

[0033] the endless belt comprising a layer containing a thermoplasticresin having a diphenyl sulfone structure represented by the followingFormula (1)

BRIEF DESCRIPTION OF THE DRAWINGS

[0034]FIG. 1 is a schematic cross-sectional illustration of an exampleof an image forming apparatus making use of the endless belt of thepresent invention as an intermediate transfer member.

[0035]FIG. 2 is a schematic cross-sectional illustration of an exampleof an image forming apparatus making use of the endless belt of thepresent invention as a transfer material carrying belt.

[0036]FIG. 3 is a schematic side elevation of an example of an extrusionapparatus for producing the endless belt of the present invention.

[0037]FIG. 4 is a partial cross-sectional perspective illustration of anintermediate transfer belt having a double-layer configuration accordingto the present invention.

[0038]FIG. 5 is a perspective illustration of an intermediate transferbelt having a triple-layer configuration according to the presentinvention.

[0039]FIG. 6 is a partial cross-sectional perspective illustration of anintermediate transfer belt having a triple-layer configuration accordingto the present invention.

[0040]FIG. 7 is a schematic perspective illustration of another exampleof an extrusion apparatus for producing the endless belt of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] The endless belt of the present invention is obtainablecontinuously by melt extrusion from a circular die and also has a layercontaining a thermoplastic resin having a diphenyl sulfone structurerepresented by the following Formula (1)

[0042] The reason why the present invention can be effective as statedpreviously will be set forth below.

[0043] In order for endless belts to less cause the spots around lineimages and satisfy the durability against repeated use as statedpreviously, they are required to have a high tensile modulus ofelasticity and a high breaking strength, and also required to have acreep resistance not to cause any change in size in the peripheraldirection even under application of a tension for a long term.

[0044] To attain such characteristics, materials constituting theendless belt and how to produce the endless belt are both veryimportant.

[0045] As a means for satisfying such characteristics, the presentinventors have discovered that it is most suitable to extrude thethermoplastic resin having a diphenyl sulfone structure represented bythe above Formula (1), into an endless belt in the manner mentionedabove. The resin having a diphenyl sulfone structure has good modulus ofelasticity, breaking strength, creep resistance and heat resistance, andalso have a flame retardancy at a high level. Then, the extrusion ofthis material by the production process of the present invention bringsabout an improvement of characteristics and can achieve very goodperformances required as endless belts such as the intermediate transferbelt, the transfer material carrying belt and the photosensitive belt.More specifically, the thermoplastic resin having a diphenyl sulfonestructure represented by Formula (1) is melt-extruded and issimultaneously stretched. Hence, the product can be made thin-gage withease without damaging the good properties inherent in the resin itself,bringing about the effect of cost reduction attributable to the materialusable in a smaller quantity, the effect of more improvement in strengthon account of the stretching, and the effect of less causing the uneventhickness and uneven resistance. Especially with regard to the colormisregistration and spots around line images, these can greatlyeffectively be prevented on account of the thin-gage belt and theimprovement in tensile modulus of elasticity. Also, since the endlessbelt can be produced continuously, the production process can be madesimple and efficient, promising a very high effect of process costreduction.

[0046] Thus, according to the present invention, a seamless endless belthaving a high electrical resistance stability, a high durability and ahigh creep resistance and being not causative of the spots around lineimages and the firing or smoking in an abnormal condition, having a highsafety, can be obtained at a low cost, and all the problems discussedpreviously can be solved. Using this endless belt, an intermediatetransfer belt, a transfer material carrying belt and a photosensitivebelt which have good characteristics can be obtained. Incidentally, inthe case of the photosensitive belt, the endless belt of the presentinvention is used as a substrate.

[0047] The endless belt of the present invention may preferably have athickness ranging from 40 μm to 300 μm. If it has a thickness smallerthan 40 μm, its extrusion stability may lower to tend to cause uneventhickness and also tend to result in an insufficient durability andstrength, so that the endless belt may break or crack in some cases. Ifon the other hand it has a thickness larger than 300 μm, the material isin a large quantity, resulting in a high cost and also a greatdifference in peripheral speed between the outer surface and innersurface of the endless belt at its portions put over the shafts to tendto cause spots around line images seriously. Moreover, the endless beltmay have so excessively a high rigidity as to require a high drivingtorque, bringing about problems that the main body must be madelarge-size and involves a cost increase.

[0048] An embodiment of a process for producing the endless belt of thepresent invention will be described below. Embodiments are not limitedto this process.

[0049]FIG. 3 shows an extrusion apparatus for producing the endless beltof the present invention. This apparatus consists basically of anextruder, an extruder die and a gas blowing unit. As shown in FIG. 3,the apparatus has extruders 100 and 110 so that a belt of double-layerconfiguration can be extruded. In the present invention, however, atleast one extruder may be provided.

[0050] A single-layer endless belt can be produced by a processdescribed below. First, an extrusion resin [the thermoplastic resinhaving a diphenyl sulfone structure represented by Formula (1)], aconducting agent and additives are premixed under the desiredformulation and thereafter kneaded and dispersed to prepare an extrusionmaterial, which is then put into a hopper 120 installed to the extruder100. The extruder 100 has a preset temperature, extruder screwconstruction and so forth which have been so selected that the extrusionmaterial may have a melt viscosity necessary for enabling the extrusioninto an endless belt in the post step and also the materials can bedispersed uniformly one another.

[0051] The extrusion material is melt-kneaded in the extruder 100 into amelt, which then enters a circular die 140. The circular die 140 isprovided with a gas inlet passage 150. Through the gas inlet passage150, a gas is blown into the circular die 140, whereupon the melt havingpassed through the circular die 140 in a tubular form inflates whilescaling up in the diametrical direction. Since the diameter is enlarged,this extrusion is called blown-film extrusion (i.e., inflation). Theblown-film extrusion enables extrusion into thin films with ease, and isalso readily achievable of an improvement in strength attributable tochanges in orientation of resin, called a stretch effect. Thus, this isparticularly preferred as a production process used in the presentinvention.

[0052] The gas to be blown here may be selected from air, nitrogen,carbon dioxide and argon. The extruded product having thus inflated intoa cylinder is drawn upward while being cooled by a cooling ring 160. Atthis stage, the extruded product passes through the space defined by adimension stabilizing guide 170, so that its final shape and dimensionsare determined. This product is further cut in desired width, thus aseamless endless belt 190 of the present invention can be obtained.

[0053] The foregoing description relates to a single-layer belt. In thecase of the endless belt of double-layer configuration, an extruder 110is additionally provided as shown in FIG. 3. Simultaneously with thekneaded melt held in the extruder 100, a kneaded melt in the extruder110 is sent to a double-layer circular die 140, and the two layers arescale-up inflated simultaneously, thus a double-layer belt can beobtained.

[0054] In the case of triple- or more layers, the extruder may beprovided in the number corresponding to the number of layers. Examplesof the endless belt of double-layer configuration consisting of a firstlayer 201 and a second layer 202 and that of triple-layer configurationconsisting of a first layer 201, a second layer 202 and a third layer203 are shown in FIG. 4, and FIGS. 5 and 6, respectively. Thus, thepresent invention makes it possible to extrude not only endless belts ofsingle-layer configuration but also those of multi-layer configurationin a good dimensional precision through one step and also in a shorttime. The fact that the extrusion can be made in a short time wellsuggests that mass production and low-cost production can be made.

[0055] In the case when the endless belt has a multi-layerconfiguration, at least one layer may contain the thermoplastic resinhaving a diphenyl sulfone structure represented by Formula (1).

[0056]FIG. 7 shows another extrusion apparatus for producing the endlessbelt of the present invention. The extrusion material put into a hopper120 is melt-kneaded in an extruder 100 and extruded from a circular die141. The melt thus excluded into a cylinder is stretched while beingtensed by a take-off mechanism (not shown) provided on the extensionline of a cooling mandrel 165. It comes into contact with the inner wallof the cooling mandrel 165 in the form of a cylindrical film havingsubstantially the desired thickness and diameter, to become cool andsolidify, followed by cutting. Thus, a seamless endless belt 190 of thepresent invention can be obtained.

[0057] The thickness of the cylindrical film thus extruded maypreferably be smaller than the width of a gap (slit) of the circulardie. Stated specifically, the former may preferably be not larger than⅓, and particularly preferably not larger than ⅕, of the latter asthickness ratio.

[0058] Similarly, the diameter proportion between the circular die andthe extruded cylindrical film, i.e., the proportion of external diameterof the cylindrical film at the time it has reached a shape dimension 180after extrusion with respect to external diameter of the die slit of thecircular die 140 or 141 may preferably be within the range of from 50%to 400% as external diameter proportion.

[0059] These values represent the state of stretch of the material. Ifthe thickness ratio is larger than ⅓, the film tends to stretchinsufficiently, tending to cause difficulties such as low strength,uneven resistance and uneven thickness. As for the external diameterproportion, if it is more than 400% or less than 50%, the film hasstretched in excess, resulting in a low production stability or makingit difficult to ensure the thickness necessary for the presentinvention. In the present invention, the blown-film extrusion(inflation) is preferred as stated previously. From this point of view,the external diameter of the resultant belt may preferably be from morethan 100% to 400% or less, and particularly preferably from 105% to400%, of the die slit external diameter of the circular die used.

[0060] In the endless-belt production process of the present invention,in order to attain the desired dimensions by scale-up inflating theextruded product while blowing air, or by drawing it under applicationof a tension, the extrusion material may preferably have a breakingextension of 2.0% or more and a tensile breaking strength of 40 MPa orabove. If the material has a breaking extension less than 2.0%, theextruded product may instantaneously solidify when it is shifted to acooling step from a molten state after it has passed through the step ofextrusion, so that it may be scale-up inflated to the desired dimensionswith difficulty. Also, if the material has a tensile breaking strengthbelow 40 MPa, the extruded product may have no body and can not maintainthe cylindrical shape at the time of scale-up inflation, tending to casewrinkles, strain and unevenness when it is drawn upward while beingscale-up inflated as shown in FIG. 3.

[0061] Uniformities of electrical resistance of the endless belt of thepresent invention and electrical resistance of the interior of the beltare important factors for maintaining the performance of the endlessbelt.

[0062] In the case of the intermediate transfer belt, if the transferbelt has a too high-electrical resistance, a sufficient transferelectric field can not be imparted at the time of primary transfer andsecondary transfer, tending to result in faulty transfer. If on theother hand it has a too low electrical resistance, electrical dischargemay locally occur, also making it hard to form the transfer electricfield. Also, if the resistance in the belt is non-uniform, the localelectrical discharge, i.e., leak may occur, and electric currentsapplied at the time of primary transfer and secondary transfer mayescape therethrough to make it hard to obtain the necessary transferelectric field. In the case of transfer making use of the transfermaterial carrying belt, too, the same as the foregoing may apply. Also,in the case of the photosensitive belt, a too high electrical resistancetends to cause a problem of a rise of residual potential.

[0063] Accordingly, according to the present invention, the endless beltmay preferably have a resistance of from 1×10⁰ to 1×10¹⁴Ω. Also, inorder to prevent such leak, faulty transfer and local uneven transferfrom occurring, the difference in resistance at every part of theendless belt may preferably be within 100 times (maximum value/minimumvalue) in respect of both the surface-direction resistance and thethickness-direction resistance.

[0064] The chief material resin included in extrusion materials used inthe endless belt of the present invention contains as its constituentmaterial at least the thermoplastic resin having a diphenyl sulfonestructure represented by the following Formula (1)

[0065] The thermoplastic resin having such a structure may preferablyinclude, but not particularly limited to, polysulfones having astructural unit represented by the following Formula (2) and polyethersulfones having a structural unit represented by the following Formula(3). Also, any of these resins having a diphenyl sulfone structure maybe used in plurality in the form of a mixture.

[0066] In the present invention, additional resin(s) may optionally bemixed in addition to the above resin. In such an instance, the resinhaving a diphenyl sulfone structure may be held in a proportion of 30%by weight or more, and more preferably 50% by weight or more, of thewhole resins. If it is in a too small proportion, the present inventioncan not be well effective in some cases.

[0067] There are no particular limitations on the additional resin(s)mixable in the endless belt of the present invention. Preferred arethose having melting temperature close to that of the resin having adiphenyl sulfone structure.

[0068] In the present invention, in order to control the electricalresistance of the endless belt, a conductive agent may be added as longas the present invention can be effective. Carbon black is commonlyused, but not necessarily limited to it. Besides, the conductive agentmay include conductive metal oxides, metal salts, and conductivemacromolecules.

[0069] Taking account of extrusion performance and mechanical propertiesof the endless belt, the conductive agent may be added in an amount of30% by weight or less based on the weight of the resins. This, however,does not necessarily apply when the conductive agent has a largedensity. In an instance where the resistance is controlled with theresin material itself, its amount is not limitative to the foregoing.

[0070] Methods of measuring physical properties concerning the presentinvention are shown below.

[0071] Tensile breaking strength:

[0072] The tensile break strength and breaking extension are measuredaccording to JIS K7113 and JIS K7127, in conformity with the nature ofthe extrusion material and the resin used in the extrusion material.

[0073] Resistance:

[0074] As measuring equipments, an ultra-high resistance meter R8340A(manufactured by Advantest Co.) is used as a resistance meter, andSample box TR42 for ultra-high resistance measurement (manufactured byAdvantest Co.) as a sample box. The main electrode is 25 mm in diameter,and the guard-ring electrode is 41 mm in inner diameter and 49 mm inouter diameter.

[0075] A sample is prepared in the following way. First, the endlessbelt is cut in a circular of 56 mm in diameter by means of a punchingmachine or a sharp knife. The circular cut piece obtained is fitted, onits one side, with an electrode over the whole surface by forming aPt—Pd deposited film and, on the other side, fitted with a mainelectrode of 25 mm in diameter and a guard electrode of 38 mm in innerdiameter and 50 mm in outer diameter by forming Pt—Pd deposited films.The Pt—Pd deposited films are formed by carrying out vacuum depositionfor 2 minutes using Mild Sputter E1030 (manufactured by Hitachi Ltd.).The one on which the vacuum deposition has been carried out is used as ameasuring sample.

[0076] Measured in a measurement atmosphere of 23° C./55% RH. Themeasuring sample is previously kept left in the like atmosphere for 12hours or longer. Measurement is made under a mode of discharge for 10seconds, charge for 30 seconds and measurement for 30 seconds and at anapplied voltage of 1 to 1,000 V.

[0077] The applied voltage may arbitrarily be selected within the rangeof from 1 to 1,000 V which is magnitude of the voltage applied when theendless belt is actually used in an image forming apparatus. It may beselected in accordance with the resistance value, thickness andinsulation breakdown strength of the sample. Also, as long as theelectrical resistance at a plurality of spots, measured at any one-pointvoltage of the above applied voltage, is included in the resistancerange defined in the present invention, the resistance is judged to bewithin the resistance range intended in the present invention.

[0078] An example of an image forming apparatus employing the endlessbelt of the present invention as an intermediate transfer member isschematically shown in FIG. 1.

[0079] The apparatus shown in FIG. 1 is a full-color image formingapparatus (copying machine or laser beam printer) utilizing anelectrophotographic process.

[0080] Reference numeral 1 denotes a drum-shaped electrophotographicphotosensitive member (hereinafter “photosensitive drum”) serving as afirst image bearing member, which is rotatingly driven at a prescribedperipheral speed (process speed) in the direction of an arrow.

[0081] The photosensitive drum 1 is, in the course of its rotation,uniformly charged to prescribed polarity and potential by means of aprimary charging assembly 2, and then exposed to light 3 by a exposuremeans (not shown; e.g., a color-original imagecolor-separating/image-forming optical system, or a scanning exposuresystem comprising a laser scanner that outputs laser beams modulated inaccordance with time-sequential electrical digital pixel signals ofimage information). Thus, an electrostatic latent image is formed whichcorresponds to a first color component image (e.g., a yellow colorcomponent image) of the intended color image.

[0082] Next, the electrostatic latent image is developed with afirst-color yellow developer (toner) Y by means of a first developingassembly (yellow color developing assembly 41). At this stage, second tofourth developing assemblies (magenta color developing assembly 42, cyancolor developing assembly 43 and black color developing assembly 44)each stand unoperated and do not act on the photosensitive drum 1, andhence the first-color yellow toner image is not affected by the secondto fourth developing assemblies.

[0083] An intermediate transfer belt 20 is rotatingly driven at aprescribed peripheral speed in the direction of an arrow. Thefirst-color yellow toner image formed and held on the photosensitivedrum 1 passes through a nip formed between the photosensitive drum 1 andthe intermediate transfer belt 20, in the course of which it issuccessively intermediately transferred to the periphery of theintermediate transfer belt 20 (primary transfer) by the aid of anelectric field formed by a primary transfer bias applied to theintermediate transfer belt 20 through a primary transfer roller 62. Thephotosensitive drum 1 surface from which the first-color yellow tonerimage has been transferred is cleaned by a cleaning assembly 13.

[0084] Subsequently, the second-color magenta toner image, thethird-color magenta toner image and the fourth-color black toner imageare sequentially similarly transferred superimposingly onto theintermediate transfer belt 20. Thus, the intended full-color toner imageis formed.

[0085] Reference numeral 63 denotes a secondary transfer roller, whichis provided in such a way that it is axially supported in parallel to asecondary transfer opposing roller 64 and stands separable from thebottom surface of the intermediate transfer belt 20.

[0086] The primary transfer bias for sequentially superimposinglytransferring the first- to fourth-color toner images from thephotosensitive drum 1 to the intermediate transfer belt 20 is appliedfrom a bias source 29 in a polarity (+) reverse to that of each toner.The voltage thus applied is, e.g., in the range of from +100 V to +2 kV.In the step of primary transfer, the secondary transfer roller 63 mayalso be set separable from the intermediate transfer belt 20.

[0087] The full-color toner images formed on the intermediate transferbelt 20 are transferred to a second image bearing member, transfermaterial P, in the following way: The secondary transfer roller 63 isbrought into contact with the intermediate transfer belt 20 andsimultaneously the transfer material P is fed at a prescribed timingfrom a paper feed roller 11 through a transfer material guide 10 untilit reaches a contact nip formed between the intermediate transfer belt20 and the secondary transfer roller 63, where a secondary transfer biasis applied to the secondary transfer roller 63 from a power source 28.The transfer material P to which the toner images have been transferredare guided into a fixing assembly 15 and are heat-fixed there.

[0088] After the toner images have been transferred to the transfermaterial P, a charging member 7 for cleaning is brought into contactwith the intermediate transfer belt 20, and a bias with a polarityreverse to that of the photosensitive drum 1 is applied, whereuponelectric charges with a polarity reverse to that of the photosensitivedrum 1 are imparted to toners not transferred to the transfer material Pand remaining on the intermediate transfer belt 20 (i.e., transferresidual toners). Reference numeral 26 denotes a bias power source. Thetransfer residual toners are electrostatically transferred to thephotosensitive drum 1 at the nip on the photosensitive drum 1 and thevicinity thereof, thus the intermediate transfer member (intermediatetransfer belt 20) is cleaned.

[0089] An example of an image forming apparatus employing the endlessbelt of the present invention as a transfer material carrying member isschematically shown in FIG. 2. In FIG. 2, a transfer material P iscarried on a transfer material carrying belt 12, and is transported inthe direction of an arrow shown in the drawing. At the same time,individual color toner images are sequentially transferred thereto froma photosensitive drum 1. In FIG. 2, reference numerals 1, 2, 3, 10, 11,13, 15, 41, 42, 43 and 44 and a letter symbol P denote the same as thosein FIG. 1; and 33 to 36, transfer means.

[0090] In the case when the endless belt of the present invention isused as a substrate for the photosensitive belt, there are no particularlimitations on the photosensitive layer on the substrate and othervarious means necessary for forming images, such as charging means anddeveloping means.

[0091] In the present invention, without regard to whether or not theendless belt is used as a substrate for the photosensitive belt, aphotosensitive drum containing fine powder of polytetrafluoroethylene(PTFE) in at least its outermost layer may preferably be used because ahigher transfer efficiency can be achieved. This is presumably becausethe incorporation of PTFE lowers surface energy of the photosensitivedrum outermost layer to bring about an improvement of releasability ofthe toner.

[0092] The present invention will be described below in greater detailby giving Examples. In the following Examples, “part(s)” is part(s) byweight.

EXAMPLE 1

[0093] Polysulfone 100 parts Conductive carbon black  16 parts

[0094] The above materials were kneaded by means of a twin-screwextruder, and the additive such as carbon black was well uniformlydispersed in the binder so as to provide the desired electricalresistance, thus an extrusion material (1) was obtained in the form ofpellets of about 2 mm in diameter. Next, this extrusion material (1) wasput into the hopper 120 of the single-screw extruder 100 shown in FIG.3, and was extruded with heating to form a melt. The melt wassubsequently brought to the circular die 140 for extruding a cylindricalsingle-layer product, having a diameter of 120 mm and a die gap of 1 mm.Then, air was blown from the gas inlet passage 150 while extruding themelt from the die, to scale-up inflate the extruded product into acylindrical extruded product of 190 mm in diameter and 160 μm inthickness as final shape dimensions 180. This product was further cut ina belt width of 320 mm to obtain a seamless endless belt typeintermediate transfer belt 190. This is designated as intermediatetransfer belt (1).

[0095] The electrical resistance of this intermediate transfer belt (1)under application of 100 V was 2×10⁵Ω. Also, the electrical resistanceof the intermediate transfer belt (1) was measured at four spots in itsperipheral direction and at two spots in its axial direction at eachposition of the former, eight spots in total, and any scattering ofelectrical resistance in one endless belt was examined, where thescattering of measurements at the eight spots was within one figure inrespect of both the surface-direction resistance and thethickness-direction resistance. Scattering in the measurement ofthickness at the same positions was within 160 μm plus-minus 15 μm. Uponvisual observation of the intermediate transfer belt (1), none offoreign matter or faulty extrusion such as granular structure and fisheyes was seen on its surface. Also, the tensile break strength andbreaking extension of the extrusion material (1) were 75 MPa and 10%,respectively.

[0096] The intermediate transfer belt (1) obtained was set in thefull-color image forming apparatus shown in FIG. 1. Using two colortoners of cyan-magenta and cyan-yellow, respectively, blue and greencharacter images and line images were printed on 80 g/m² paper in anenvironment of 23° C./60% RH.

[0097] The respective images were visually observed to make evaluationon color misregistration and spots around line images. As a result,there were no problems on the both, showing good results.

[0098] Next, an A4 full-color image 50,000-sheet continuous running testwas made while cleaning the intermediate transfer belt by acleaning-at-primary-transfer method in which electric charges having apolarity reverse to the normal charge were imparted to the secondarytransfer residual toners to return them to the photosensitive member.

[0099] After the running, very slight spots around line images and colormisregistration were seen compared with initial-stage images but werenot particularly problematic, and good images were obtainable. Neitherfaulty images and faulty drive due to the creep nor toner filmingoccurred, and also no problems were seen on cracking, scrape, wear andso forth. Thus, the belt was judged to have a sufficient durability.

EXAMPLE 2

[0100] Polysulfone 80 parts Polyether sulfone 20 parts Conductive carbonblack 16 parts

[0101] The above materials were kneaded by means of a twin-screwextruder to obtain a uniform kneaded product, which was designated as anextrusion material (2). Next, this was continuously extruded by means ofthe extruder shown in FIG. 7, using a circular extrusion die 141 havinga diameter of 200 mm and a die gap of 1.2 mm. The cylindrical extrudedproduct obtained was cut to obtain an intermediate transfer belt (2) of185 mm in diameter, 320 mm in belt width and 125 μm in thickness.

[0102] The tensile break strength and breaking extension of theextrusion material (2) were 80 MPa and 6%, respectively. The electricalresistance of the intermediate transfer belt (2) under application of100 V was 3×10^(5Ω.)

[0103] The scattering of electrical resistance was within one figure inrespect of both the surface-direction resistance and thethickness-direction resistance. The scattering of thickness was also asgood as 125 μm plus-minus 10 μm.

[0104] Next, using this intermediate transfer belt (2), printing wastested in the same manner as in Example 1 to obtain good results likethose in Example 1.

EXAMPLE 3

[0105] Polyether sulfone 80 parts Polybutylene terephthalate 20 partsConductive carbon black 15 parts

[0106] The above materials were kneaded by means of a twin-screwextruder to obtain a uniform kneaded product, which was designated as anextrusion material (3). The subsequent procedure of Example 1 wasrepeated to obtain an intermediate transfer belt (3) of 190 mm indiameter, 320 mm in belt width and 155 μm in thickness.

[0107] The electrical resistance of this intermediate transfer belt (3)under application of 100 V was 6×10⁵Ω. The scattering of electricalresistance was within one figure in respect of both thesurface-direction resistance and the thickness-direction resistance. Thescattering of thickness was also as good as 155 μm plus-minus 11 μm. Thetensile break strength and breaking extension of the extrusion material(3) were 71 MPa and 11%, respectively.

[0108] Next, using this intermediate transfer belt (3), printing wastested in the same manner as in Example 1 to obtain good results likethose in Example 1.

EXAMPLE 4

[0109] Polysulfone 100 parts Conductive carbon black  10 parts

[0110] The above materials were kneaded and dispersed in the same manneras in Example 1 to obtain an extrusion material (4) in the form ofpellets of about 2 mm in diameter. The subsequent procedure of Example 1was repeated except for using a circular extrusion die having a diameterof 200 mm and a die gap of 0.6 mm, to obtain a transfer materialcarrying belt (1) of 280 mm in diameter, 250 mm in belt width and 150 μmin thickness.

[0111] This transfer material carrying belt (1) had an electricalresistance of 8×10¹¹Ω under application of 100 V. Its scattering ofthickness was 150 μm plus-minus 24 μm, and scattering of electricalresistance was within one figure in respect of both thesurface-direction resistance and the thickness-direction resistance. Thetensile break strength and breaking extension of the extrusion material(4) were 72 MPa and 12%, respectively.

[0112] This transfer material carrying belt was set in the apparatusshown in FIG. 2, and printing was tested in the same pattern and manneras in Example 1.

[0113] After the running, very slight spots around line images and colormisregistration were seen compared with initial-stage images but werenot particularly problematic, and good images were obtainable. Neitherfaulty images and faulty drive due to the creep nor toner filmingoccurred, and also no problems were seen on cracking, scrape, wear andso forth. Thus, the belt was judged to have a sufficient durability.

COMPARATIVE EXAMPLE 1

[0114] Low-density polyethylene 100 parts Conductive carbon black  15parts

[0115] The above materials were kneaded and dispersed by means of atwin-screw extruder to obtain a uniform kneaded product, which wasdesignated as an extrusion material (5). The subsequent procedure ofExample 1 was repeated to obtain an intermediate transfer belt (4) of190 mm in diameter, 320 mm in belt width and 140 μm in thickness.

[0116] The electrical resistance of this intermediate transfer belt (4)under application of 100 V was 6×10⁶Ω. The scattering of electricalresistance was within one figure in respect of both thesurface-direction resistance and the thickness-direction resistance. Thescattering of thickness was 140 μm plus-minus 38 μm. The tensile breakstrength and breaking extension of the extrusion material (5) were 30MPa and 250%, respectively.

[0117] Next, using this intermediate transfer belt (4), printing wastested in the same manner as in Example 1. As a result, both the colormisregistration and the spots around line images occurred seriously atthe initial stage. The color misregistration and spots around lineimages became more serious with progress of running and also unevenimages occurred. Hence the running test was stopped on 10,000th sheet.Thus, this intermediate transfer belt was found to have insufficientstrength and durability.

What is claimed is:
 1. A process for producing an endless belt forelectrophotography, the process comprising the step of melt-extruding athermoplastic resin having a diphenyl sulfone structure represented bythe following Formula (1) from a circular die to produce the endlessbelt continuously


2. The process according to claim 1, wherein said thermoplastic resinhaving a diphenyl sulfone structure is a thermoplastic resin having astructural unit represented by the following Formula (2) or (3)


3. The process according to claim 1, wherein said endless belt has athickness of from 40 μm to 300 μm.
 4. The process according to claim 1,wherein said endless belt has a thickness not larger than ⅓ of the slitwidth of the circular die used.
 5. The process according to claim 1,wherein said endless belt has a thickness not larger than ⅕ of the slitwidth of the circular die used.
 6. The process according to claim 1,wherein said endless belt has an external diameter of from 50% to 400%of the external diameter of the die slit of the circular die used. 7.The process according to claim 1, wherein said endless belt has anexternal diameter of from more than 100% to 400% or less of the externaldiameter of the die slit of the circular die used.
 8. The processaccording to claim 1, wherein said endless belt has an external diameterof from 105% to 400% of the external diameter of the die slit of thecircular die used.
 9. The process according to claim 1, wherein saidendless belt has a resistance of from 1×10⁰Ω to 1×10^(14Ω.)
 10. Theprocess according to claim 1, wherein said endless belt has a maximumvalue of a surface-direction resistance that is not greater than 100times a minimum value of said surface-direction resistance.
 11. Theprocess according to claim 1, wherein said endless belt has a maximumvalue of a thickness-direction resistance that is not greater than 100times a minimum value of said thickness-direction resistance.
 12. Theprocess according to claim 1, wherein said endless belt is anintermediate transfer belt.
 13. The process according to claim 1,wherein said endless belt is a transfer material carrying belt.
 14. Theprocess according to claim 1, wherein a gas is blown to the inside of acylindrical film of the thermoplastic resin melt-extruded from thecircular die, to make the endless belt have an external diameter largerthan the external diameter of the die slit of the circular die.
 15. Theprocess according to claim 1, wherein an extrusion material to bemelt-extruded, which contains the thermoplastic resin having a diphenylsulfone structure, has a breaking extension of 2% or more.
 16. Theprocess according to claim 1, wherein an extrusion material to bemelt-extruded, which contains the thermoplastic resin having a diphenylsulfone structure, has a tensile breaking strength of 40 MPa or above.